1. Field of the Invention
The present invention relates generally to multilayer film constructions. More particularly the present invention relates to high vapor and gas barrier multilayer film constructions comprising at least one fluoropolymer film, at least one thermoplastic polymer film, and an adhesive layer between each film.
2. Description of the Prior Art
A wide variety of thermoplastic resins are presently known, and films formed from such thermoplastic resins feature chemical and physical characteristics which are related to the thermoplastic resin from which they are formed. For the purposes of the present invention, the term film includes sheets as well as films, unless otherwise indicated.
Important physical characteristics of films which are of significant interest to the art include the barrier properties, including gas, aroma, and/or vapor such as water vapor, of the film as well as the physical characteristics, such as toughness, wear and weathering resistances, and light-transmittance. Such properties are especially important in the film applications wherein such properties are critical; one example of which includes the use of films as a packaging material for foodstuffs or medicaments.
Fluoropolymers are a class of paraffinic polymers that have some or all of the hydrogen replaced by fluorine, and they are known in the art for their inertness to most chemicals and resistance to high temperatures as well as low coefficients of friction. Most fluoropolymers, especially polychlorotrifluoroethylene (PCTFE) and ethylene-chlorotrifluoroethylene (ECTFE), exhibit excellent barrier properties, making them exceptionally good polymers as a barrier packaging material. However, such use of fluoropolymers is restricted to specialty packaging applications due to their relatively high cost.
A suitable means of reducing the cost of a packaging material fabricated from a costly polymer is to form multilayer structures in which the polymer film is laminated with other, less costly polymer films. This approach is particularly desirable for the fluoropolymer packaging applications since a thin layer of the fluoropolymer is often all that is needed to take advantage of the desirable properties of fluoropolymers while minimizing the cost. However, fluoropolymers do not adhere strongly to most other polymers; in fact, most fluoropolymers are known for their non-stick characteristics.
Various attempts have been made to produce such multilayer fluoropolymer film structures. One such film structure is described in U.S. Pat. No. 4,677,017 to DeAntonis et al. Such patent in particular discloses coextruded multilayer films which include at least one fluoropolymer film and at least one thermoplastic film which are joined by the use of an adhesive polymer, particularly ethylene/vinyl acetate resins, as an adhesive layer. U.S. Pat. No. 4,659,625 to Decroly et al. discloses a fluoropolymer multilayer film structure which utilizes a vinyl acetate polymer as an adhesive layer.
There remains a continuing need in the art for further improvements in fluoropolymer films and film structures, particularly those which provide a film structure featuring low water vapor and gas transmission, and good physical characteristics.